The isolated perfused rat kidney will be used to study the relationship between adenylate cyclase, hydrogen ion secretion mediated by carbonic anhydrase, and changes in tubular transport of ions affected by parathyroid hormone (PTH). This preparation transports solutes at physiologic rates and permits independent control of hydrogen ion (H ion), calcium (Ca2 ion), and phosphate (HPO4 ion) ccncentrations to determine their role as possible modulators of the action of PTH. Absolute and fractional excretion of sodium, potassium, bicarbonate, calcium and phosphate will be measured by clearance techniques and correlated with changes in cyclic 3'-5'-adenosine monophosphate (cyclic AMP) excretion under a variety of experimental conditions. Metabolism of PTH by the kidney will also be related to these events. Interrelationships between carbonic anhydrase, H ion secretion, and adenylate cyclase will be studied by administrating a carbonic anhydrase inhibitor, acetazolamide, or PTH to kidneys perfused either in the presence of bicarbonate or its absence when hydrogen ion secretion is limited by low filtered buffer. The requirements for expression of PTH action by the simultaneous presence of vitamin D3 metabolites will be tested. The roles of hydrogen ion and calcium will be studied by varying the bicarbonate concentration from 12.5 to 40 mM and PCO2 from 20 to 80 mM Hg with corresponding variation of pH from 7.10 to 7.75 or alternatively by maintaining pH constant and varying free calcium from 0.5 to 2.0 mM. An ionophore, A23187, can be used to increase influx of calcium into cells to determine whether intracellular calcium directly affects ion transport or modulates the activation of adenylate cyclase by PTH. Phosphate can be varied from 1.0 to 5.0 mM to elucidate its effects.